Low-density carbon-bonded carbon fiber (CBCF) composite is a versatile material originally developed at the Oak Ridge National Laboratory for thermal insulation. It consists of randomly-oriented chopped carbon fibers bonded by glassy carbon such that the bulk density is typically 10% of theoretical density.
Two other applications for low-density CBCF composite were as a light absorbing article and as a light emitting article. (U.S. Pat. Nos. 5,243,464, issued Sep. 7, 1993; and 5,313,325, issued May 17, 1994.) More recently, we have applied CBCF composite as a coating on the internal copper surfaces of microwave tubes resulting in improved tube performance (see our co-pending Application entitled "Improved Microwave Tube"). We have also produced shaped bodies of CBCF composite and applied them as attenuators (i.e., load elements) in microwave waveguides and coaxial transmission lines (our co-pending Application entitled "Improved Radio-Frequency and Microwave Load").
All of the above uses for CBCF composites (i.e., optical absorbers, optical emitters, microwave tube coatings, and microwave attenuators) would be enhanced if a suitable method for joining CBCF articles to metals, particularly copper, were available.
In our optical absorber patent, U.S. Pat. No. 5,243,464, we disclosed a method for directly bonding CBCF composite to carbonaceous material. As it turns out, the method is appropriate for a graphite substrate but not for attachment to metals such as copper because the high temperature required for the carbonization step is above the melting point of copper.
Organic adhesives are another candidate bonding method. Organic adhesives, however, are of limited value in applications that require a very clean article or use of high vacuum. High-vacuum systems such as microwave tubes and sensitive optical systems cannot tolerate components such as adhesives that outgas during service. Also, microwave waveguides and coaxial transmission lines require good thermal and electrical contact between the CBCF composite and the metal substrate they are attached to. Organic adhesives are not suitable for these purposes either.
We also tried vacuum brazing. Our attempts to use braze alloys to vacuum braze CBCF composite to copper and other metals lacked success because the molten braze alloy "wicked" into the CBCF composite rather than bond the CBCF composite to the metal substrate.